Antagonism of Pseudomonas putida Strain PP22 to Phytopathogenic Bacteria and Its Potential Use as a Biocontrol Agent. C. H. Liao, Research Plant Pathologist, Eastern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, Philadelphia, PA 19118. . Plant Dis. 73:223-226. Accepted for publication 17 October 1988. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1989. DOI: 10.1094/PD-73-0223.

Twenty-seven of 963 strains of epiphytic bacteria isolated from fruits of bell pepper and tomato were found to be antagonistic to Erwinia carotovora subsp. carotovora on KB medium. All but two of the antagonistic strains were characterized and identified: eight as Pseudomonas fluorescens, 16 as P. putida, and one as Flavobacterium sp. One strain of P. putida, PP22, inhibited the growth of a broad spectrum of phytopathogenic bacteria on media, including 24 strains of soft rot bacteria (in the genera of Erwinia, Pseudomonas, Xanthomonas, and Cytophaga), one strain of P. solanacearum, four pathovars of X. campestris, and five pathovars of P. syringae. Strain PP22 produced iron-chelating siderophores and an antibacterial compound that was heat- and trypsin-resistant. This strain suppressed the growth of E. c. subsp. carotovora on potato slices and survived on the tubers and roots of potato plants for more than 5 wk. Application of strain PP22 to potato tubers reduced the severity of bacterial soft rot caused by E. c. subsp. carotovora and X. campestris by an average of 21 and 44%, respectively, in four separate trials.